I have been playing with my Raspberry Pi, turning it into a Pwn Plug and such, and started trying to think of a good home-defense use for it.
Enter Labrea (http://linux.die.net/man/1/arp-scan)
Labrea responds to ping scans, port scans, and such, and even throttles the traffic to slow down potential hackers/malware. It also stores its results in the /var/log/syslog, acting like a mini IDS! But who wants to use a full-blown machine to do something so simple? This sort of activity is PERFECT for the Raspberry Pi. Low power, low noise, and inconspicuous.
With a simple apt-get install labrea, I was up and running. True, I had done a bunch of tweaking prior to that, so if that single command doesn't work for you, then I'm sorry I didn't document all my efforts.
Running Labrea was as simple as: #labrea -v -i eth0 -sz -d -n network/CIDR
I ran a ping scan of my network, and Labrea dutifully responded everywhere. I ran a NMap scan of a host, and it had thousands of ports open (on a host that wasn't even there). It is truly a fun way to mess with your network, and with a Pi you can mess very cheaply!